(1) Field of the Invention
The present invention relates to an equalizing and amplifying circuit in an optical signal receiving apparatus, and more particularly to an optical receiving apparatus suitable for circuit integration and having an ability to output a signal of constant and equalized amplitude.
In an optical communication system having a common trunk line, various services such as data communication or picture communication have been developed. Accompanied by those developments, high speed and long distance communication have been desired. An optical signal receiving apparatus in such an optical transmission system has an optoelectric converter for converting an optical signal received through a long distance transmission line into an electrical signal, and an equalizing and amplifying circuit for identifying the received signal. It is desired that the equalizing and amplifying circuit have characteristics of an extremely wide band width and low noise, because the current of the super high speed signal from a light-receiving element is very small due to the long distance transmission.
In addition, when the optical signal receiving apparatus is made as an integrated circuit (IC) and is mounted in a package, it is necessary to suppress the influence of parasitic inductances, caused by bonding wires between the IC chip and the package, on the characteristics of the circuit. Therefore, it is important to design the circuit in such a way that the circuit and the mounting should not separately be designed but should be designed as a single unit so that the mounting does not influence the characteristics of the circuit.
Further, the amplitudes of the optical signals transmitted through optical fibers to the optical signal receiving part are different depending on the transmission lines, deterioration of an optical signal transmitting part over several years, and so forth. It is desired, however, to output a signal of constant equalized amplitude regardless of whether the input signal amplitude is large or small.
In general, the prestage part of such an equalizing and amplifying circuit as mentioned above is constructed by a preamplifier and an automatic gain control circuit. The preamplifier circuit amplifies an electrical signal, obtained by converting an optical signal, to a predetermined signal level. The automatic gain control circuit compensates the signal levels output from the preamplifier circuit to output a signal of a predetermined level.
(2) Description of the Prior Art
Conventionally, as the preamplifier circuit, a transimpedance-type common-emitter amplifying circuit is used. The operation of this conventional circuit, however, is easily influenced by fluctuations in power supply voltage. Accordingly, the operation is easily influenced by inductances of bonding wires between the ground pattern on the package and the circuit, or between the power supply pattern on the package and the circuit, as later described in more detail with reference to the drawings.
Further, conventionally, to obtain a constant output amplitude of the equalized signal in response to a variation of the optical signal input amplitude, the multiplication factor of the light receiving element is controlled by the feedback from the output of the equalizing and amplifying circuit, the output gain of the circuit is controlled by means of an automatic gain control circuit (hereinafter referred to as an AGC circuit), or the output gain of the circuit is controlled by the combination of the above two methods.
With the method of controlling the multiplication factor of the light receiving element, however, there is a disadvantage in that the gain frequency characteristic for a small amplitude signal is changed depending on the multiplication factor. Therefore, the waveform of the output signal is changed depending on the amplitude of the input signal so that an equalized output having a gain constant amplitude cannot be obtained.
With the method of controlling the gain of the AGC circuit, there is a disadvantage in that the amplitude of the input signal to the AGC circuit is limited in order to linearly operate the AGC circuit so that a sufficient dynamic range cannot be obtained.
These disadvantages of the impossibility of maintaining a constant equalized output amplitude and insufficient dynamic range are caused by controlling the multiplication factor of the light receiving element or by controlling the gain of the AGC circuit. Accordingly, it has been desired to provide a preamplifier circuit the gain of which can be directly controlled.